1. Field of the Invention
This invention relates to a pneumatic radial tire, and more particularly to a pneumatic radial tire for passenger car having high running-performances and provided with a directional tread pattern specifying a rotating direction of the tire when mounted onto a vehicle. Many blocks are formed in the trend pattern at given intervals in a circumferential direction of the tire by a plurality of circumferential grooves arranged at given intervals in an axial direction of the tire and extending in parallel to or substantially in parallel to the circumferential direction. A plurality of directional slant grooves arranged in the trend pattern at given intervals in the circumferential direction and obliquely extending with respect to the circumferential direction.
The term "directional slant groove" used herein means a groove obliquely extending with respect to the circumferential direction and specifying a rotating direction (forward direction) of the tire to precedently contact a portion of the slant groove near to an equatorial plane of the tire with the ground and succeedingly contact a portion of the slant groove apart from the equatorial plane with ground when the tire is mounted onto a vehicle or forming so-called directional tread pattern.
2. Description of Related Art
In the high running-performance pneumatic radial tire for a passenger car, it is required to have excellent steering stability on a dry road surface and a low level of tire noise while highly maintaining a drainage property on a wet road surface. However, these performances required in the tire conflict with each other in the pattern design, so that it is very difficult to provide a tire simultaneously establishing excellent drainage property in the running on wet road surface, steering stability in the running on dry road surface and low tire noise level according to the conventional technique.
In order to enhance the drainage property in the running on wet road surface, for instance, there is proposed and practiced a tread pattern wherein many blocks are formed at intervals in an axial direction and a circumferential direction of the tire by a plurality of circumferential grooves straightforward extending in the circumferential direction and a plurality of slant grooves obliquely extending with respect to the circumferential direction. In order to more improve the drainage property in the running on wet road surface, it is favorable to render the slant groove into a steeply slant groove extending at a small inclination angle with respect to the circumferential direction. On the other hand, in order to enhance the steering stability in the running on dry road surface, it is favorable to render the slant groove into a gently slant groove extending at a large inclination angle with respect to the circumferential direction. Particularly, when the inclination angle of the slant groove is made large with respect to the circumferential direction, there is caused a problem of deteriorating the tire noise level.